Diverse bubble generating apparatus are known. In one known apparatus, a method is employed in which a compressed gas is injected through fine holes into a liquid so as to cause growth of bubbles containing therein the compressed gas. In another known apparatus, a nozzle device is employed in which a gas or a gas-liquid mixture is directly injected from a nozzle means but no means for causing collision and dispersion of a liquid is provided. In a further known apparatus, a rotary cylindrical element rotating at a high speed in a liquid is used in which a gas is fed into the rotary cylindrical element so as to cause dispersion of fine bubbles through the wall of the rotary cylindrical element. According to any of these known apparatus, however, it is difficult to generate bubbles having sufficient fineness for attaining gas-liquid contact to a desired degree and it is also difficult to appropriately set the angle of dispersing the stream of gas bubbles or the bubble-contained liquid into the liquid. Further, when the rotary cylindrical element rotating at a high speed in a liquid is employed, provision of a shaft-sealing device is indispensable and this device involves such defects as difficult maintenance and short life.